1. Field of the Invention
The present invention relates to radio frequency preamplifiers and in particular to a pre-amplifier for use with receivers adapted to receive both frequency and amplitude modulated signals. The pre-amplifier includes means for separating the amplitude and frequency modulated signals, amplifying the frequency modulated signals only, and recombining the signals for injection into the receiver.
2. Description of the Prior Art
Pre-amplifier circuits are well known and widely used. Typically, such circuits are used to increase the magnitude of received signals in exactly the form in which they are received, that is, without any attempt to demodulate or otherwise derive the intelligence contained therein. Such circuits are of particular value for increasing the reception sensitivity of receivers adapted to demodulate frequency modulated radio signals.
It is also known to provide radio receivers which are adapted to receive both amplitude and frequency modulated radio signals, and such receivers are becoming increasingly common for automotive use. In this latter application, a pre-amplifier for frequency modulated signals is of particular value due to the obvious limitations in attaching specialized antenna systems to a moving vehicle.
However, it is also well known that an amplifier circuit and in particular a pre-amplifier circuit also receives and amplifies noise in direct proportion to the bandwidth of the pre-amplifier. Correspondingly, prior art pre-amplifiers which are adapted for use with receivers capable of demodulating both amplitude and frequency modulated signals have been adapted to receive signals over exceptionally broad bandwidths to thereby receive and pass signals in both amplitude modulated and frequency modulated frequency bands. Correspondingly, such pre-amplifiers, in addition to increasing the strength of the received frequency modulated signals, have also reduced the signal-to-noise ratio of frequency modulated signals thereby diminishing the value and effectiveness of the pre-amplifier.